Contrast improving sheet and rear projection screen provided with the same

ABSTRACT

A contrast improving sheet includes a first optical functional element and a second optical functional element on the viewing side thereof. The first optical functional element has a plurality of protrusions, each having a pair of opposite total-reflection facets for totally reflecting image light rays substantially perpendicularly incident on the entrance surface and a flat facet extending between the front edges, on the side of the exit surface, of the pair of opposite total-reflection facets. Light absorbing layers are formed between the adjacent protrusions. The second optical functional element has a light path correcting layer for correcting light paths followed by the image light rays totally reflected by the total-reflection facets such that the image light rays are emitted through the exit surface substantially perpendicularly to the exit surface. The light path correcting layer has a plurality of alternately inclined planes inclined in opposite directions.

TECHNICAL FIELD

The present invention relates to a contrast improving sheet to bedisposed on the viewing side of a display, such as a rear projectiondisplay, a liquid crystal display, a plasma display or a CRT, and to arear-projection screen provided with the same contrast improving sheet.The term “contrast improving sheet” is used in this specification tosignify an optical sheet for suppressing decrease in contrast in imagesdisplayed by a display due to external light and stray light to improvecontrast in images displayed by the display.

BACKGROUND ART

There have been proposed various techniques for optimizing viewing angleof a display, such as a rear projection display or a liquid crystaldisplay, and improving contrast in images displayed by the display sothat the viewers are able to recognize images clearly.

More concretely, a previously proposed first optical element is providedwith lenticular lenses, namely, light diffusing elements, on its backsurface (a surface facing an image light source) on which image lightfalls, and with light absorbing elements on light intercepting parts,which do not transmit image light, in its front surface on the viewingside to absorb external light incident on the front surface.

A second optical element proposed in JP 50-121753 U and JP 60159733 A isprovided on its front surface on the viewing side, as light diffusingelements, with many ribs each having inclined reflecting facets and anexit facet formed between the inclined reflecting facets, and a lightabsorbing material filling up V-grooves between the adjacent inclinedfacets to absorb external light incident on the front surface from theviewing side.

Although the first optical element is effective to some extent inimproving contrast, the first optical element cannot be used as adirectional optical element because parallel light rays perpendicularlyincident on the first optical element are diffused by the lenticularlens elements. In the first optical element, the lenticular lenselements on the back surface and the light absorbing parts on the frontsurface need to correspond to each other in one-to-one correspondence.Therefore, it is very difficult to manufacture the first optical elementwhen the lenticular lenses are arranged at short pitches.

The second optical element divides incident light rays into outgoinglight rays that are emitted through the front surface respectively inthree directions, namely, light rays not reflected by the inclinedfacets of the ribs and emitted through the exit facets in a directionnormal to the exit facets, light rays totally reflected by the oppositeinclined facets of each rib and emitted through the exit facet in twodirections at specific angles to the exit facet. Therefore, the ribsthrough which light rays travel must contain a large amount of lightdiffusing particles to display uniform images. The large amount of lightdiffusing particles spoils the clearness of images and hence the secondoptical element cannot be used as a directional optical element.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of those problems and it istherefore an object of the present invention to provide a contrastimproving sheet to be disposed in front of the front surface, on theviewing side, of a display, such as a rear projection display or aliquid crystal display, capable of increasing contrast and havingexcellent directivity, and to provide a rear projection screen providedwith the same contrast improving sheet.

A contrast improving sheet in a first aspect of the present invention tobe disposed in front of the front surface, on the viewing side, of adisplay, such as a rear projection display, a liquid crystal display ora plasma display, having an entrance surface and an exit surface,includes an optical element adapted to emit image light raysperpendicularly or substantially perpendicularly (hereinafter, referredto simply as “substantially perpendicularly”) incident on the entrancesurface in directions perpendicular or substantially perpendicular(hereinafter, referred to simply as “substantially perpendicular”) tothe exit surface regardless of incidence position on the entrancesurface and to absorb stray light rays obliquely incident on theentrance surface and external light fallen on the exit surface.

As shown in FIG. 15, the contrast improving sheet in the first aspect ofthe present invention (1) emits image light rays 4 substantiallyperpendicularly incident on the entrance surface of the contrastimproving sheet 11 through the exit surface in directions substantiallyperpendicular to the exit surface (parallel to the incident light rays),(2) absorbs external light 41 fallen on the exit surface perpendicularlyand obliquely or makes the external light 41 transmit through theentrance surface so that the external light 41 may not return toward theviewing side, and (3) absorbs stray light rays 42 fallen on the entrancesurface at incidence angles not smaller than about 30° and prevents thestray light rays 42 from traveling toward the viewing side. The contrastimproving sheet having those three functions stated in (1) to (3) iscapable of transmitting the image light fallen on the entrance surfacewithout causing loss at high directivity toward the viewing side and ofabsorbing stray light rays and external light. Differing from theconventional lenticular lens sheet, the contrast improving sheet of thepresent invention transmits image light rays toward the viewing sidewith high directivity scarcely diffusing the image light rays. Thus, thecontrast improving sheet can be used as a directional, optical element,or an optical element for a focusing system. As shown in FIG. 16, aconventional lenticular lens sheet 43 diffuses image light rays 4substantially perpendicularly incident on its entrance surface and lightrays 42 incident on the entrance surface at incident angles of about 30°or greater and travel in stray light rays through the lenticular lenssheet 43 such that diffused light rays are emitted toward the viewingside.

In the contrast improving sheet in the first aspect of the presentinvention, it is preferable that the optical element includes: a firstoptical functional element provided with a plurality of protrusionsarranged in a direction or distributed in a plane, each of theprotrusions having a pair of opposite total-reflection facets fortotally reflecting image light rays substantially perpendicularlyincident on the entrance surface thereof and a flat facet extendingbetween the front edges, on the side of the exit surface thereof, of thepair of opposite total-reflection facets, and light absorbing layersformed between the adjacent protrusions; and a second optical functionalelement attached to a surface, on the viewing side, of the first opticalfunctional element, and provided with a light path correcting layer forcorrecting the light paths followed by the image light rays totallyreflected by the total-reflection facets of the protrusions such thatthe image light rays are emitted through the exit surface thereofsubstantially perpendicularly to the exit surface thereof. Preferably,the light path correcting layer of the second optical functional elementhas a plurality of inclined surfaces formed in an alternate arrangementand capable of sending out the image light rays completely reflected bythe total-reflection facets of the first optical functional element in adirection substantially perpendicular to the exit surface.

Thus, the image light rays substantially perpendicularly incident on theentrance surface of the contrast improving sheet in the first aspect ofthe present invention, totally reflected by the total-reflection facetsof the protrusions of the first optical functional element and traveledthrough the flat facets are refracted at the inclined surfaces includedin the light path correcting layer of the second optical functionalelement and are emitted from the second optical functional elementsubstantially perpendicularly to the exit surface. The image light rayssubstantially perpendicularly incident on the entrance surface andtraveled through the flat facets without being totally reflected by thetotal-reflection facets of the protrusions of the first opticalfunctional element are totally reflected by the inclined surfacesincluded in the light path correcting layer of the second opticalfunctional element and are refracted at the inclined surfaces opposed tothe former inclined surfaces included in the light path correcting layerof the second optical functional element and are emitted from the secondoptical functional element substantially perpendicularly to the exitsurface. Thus, the light absorbing layers of the first opticalfunctional element absorb stray light rays and external light; inaddition, the light path correcting layer of the second opticalfunctional element corrects the light paths followed by the image lightrays totally reflected by the total-reflection facets of the protrusionsof the first optical functional element such that the image light raysare emitted substantially perpendicular to the exit surface toward theviewing side; and the image light rays traveled through the flat facetsof the protrusions of the first optical functional element without beingtotally reflected by the total-reflection facets of the protrusions areemitted substantially perpendicularly to the exit surface. Consequently,the contrast improving sheet achieves contrast improvement and highdirectivity without causing problems that will spoil the clearness ofimages.

In the contrast improving sheet in the first aspect of the presentinvention, it is preferable that the inclined surfaces included in thelight path correcting layer of the second optical functional element areformed such that substantially parallel are both image light rayssubstantially perpendicularly incident on the entrance surface thereof,traveled through the flat facets without being reflected by thetotal-reflection facets of the protrusions of the first opticalfunctional element, reflected by the inclined surfaces and refracted atthe inclined surfaces opposed to the former inclined surfaces, and imagelight rays totally reflected by the total-reflection facets of theprotrusions of the first optical functional element, traveled throughthe flat facets and refracted at the inclined surfaces.

In the contrast improving sheet in the first aspect of the presentinvention, it is preferable that the light absorbing layers of the firstoptical functional element are formed of a transparent material having arefractive index smaller than that of a material forming the protrusionsof the first optical functional element, the light absorbing layerscontaining light absorbing particles. In the contrast improving sheet inthe first aspect of the present invention, it is preferable that thelight absorbing layers of the first optical functional element areformed of a colored material having a refractive index smaller than thatof the material forming the protrusions of the first optical functionalelement.

The light absorbing layers effectively absorb not only external lightthat falls on the exit surface, but also the stray light rays incidenton the entrance surface at incidence angles not smaller than about 30°without totally reflecting the stray light rays by the total-reflectionfacets of the first optical functional element and hence the contrastimproving sheet is able to improve contrast.

Preferably, the contrast improving sheet in the first aspect of thepresent invention meets a condition expressed by an expression:n₃/n₂=cos 3θ₂/cos θ₂, where θ₂ is the inclination angle of the inclinedsurfaces of the light path correcting layer of the second opticalfunctional element (angle between each of the inclined surfaces and anormal to the sheet surface of the second optical functional element),n₂ is the refractive index of a material forming a part of the lightpath correcting layer on the side of the first optical functionalelement with respect to the inclined surfaces, and n₃ is the refractiveindex of a material forming a part of the light path correcting layer onthe opposite side of the first optical functional element with respectto the inclined surfaces. Thus, the image light rays substantiallyperpendicularly incident on the entrance surface and passed through theflat facets without being reflected by the total-reflection facets ofthe protrusions of the first optical functional element can be moresurely emitted perpendicularly to the exit surface (in a directionparallel to the incident light rays). Thus, the contrast improving sheetis able to achieve high directivity.

Preferably, the contrast improving sheet in the first aspect of thepresent invention meets a condition expressed by an expression: n₂·sin2θ₂=n₁·sin 2θ₁, where θ₁ is the inclination angle of thetotal-reflection facets of the protrusions of the first opticalfunctional element (angle between each of the total-reflection facetsand a normal to the sheet surface of the first optical functionalelement), n₁ is the refractive index of a material forming theprotrusions of the first optical functional element, θ₂ is theinclination angle of the inclined surfaces of the light path correctinglayer of the second optical functional element (angle between each ofthe inclined surfaces and a normal to the sheet surface of the secondoptical functional element), and n₂ is the refractive index of amaterial forming a part of the light path correcting layer on the sideof the first optical functional element with respect to the inclinedsurfaces. Thus, image light rays substantially perpendicularly incidenton the entrance surface, totally reflected by the total-reflectionfacets of the protrusions of the first optical functional element andpassed through the flat facets can be refracted at the inclined surfacesof the light path correcting layer of the second optical functionalelement and can be emitted more surely perpendicularly to the exitsurface (in a direction parallel to the incident light). Thus, thecontrast improving sheet is able to achieve high directivity.

In the contrast improving sheet in the first aspect of the presentinvention, it is preferable that the protrusions forming the secondoptical functional element are arranged in a direction and parallel tothe protrusions of the first optical functional element when theprotrusions of the first optical functional element are arranged in adirection. Also, it is preferable that the protrusions forming thesecond optical functional element are distributed in a plane parallel tothe protrusions of the first optical functional element when theprotrusions of the first optical functional element are distributed in aplane.

A rear projection screen in a second aspect of the present invention fordisplaying images by transmitting image light rays projected thereon andemitting the image light rays toward the viewing side includes: aFresnel lens sheet that deflects the image light rays incident on theentrance surface thereof toward the viewing side in a directionsubstantially perpendicular to the exit surface thereof; and thecontrast improving sheet in the first aspect of the present invention,the contrast improving sheet being disposed on the viewing side of theFresnel lens sheet.

Since the rear projection screen in the second aspect of the presentinvention includes, in combination, the Fresnel lens sheet that deflectsthe image light rays incident on the entrance surface toward the viewingside in a direction substantially perpendicular to the exit surface andthe contrast improving sheet in the first aspect of the presentinvention, it is possible to provide a rear projection screen that canincrease contrast and has high directivity.

A rear projection screen in a third aspect of the present invention fordisplaying images by transmitting image light rays projected thereon andemitting the image light rays toward the viewing side includes: aFresnel lens sheet that deflects the image light rays incident on theentrance surface thereof toward the viewing side in a directionsubstantially perpendicular to the exit surface thereof; a lenticularlens sheet disposed on the viewing side of the Fresnel lens sheet todiffuse the image light rays; and the contrast improving sheet in thefirst aspect of the present invention, the contrast improving sheetbeing disposed on the viewing side of the lenticular lens sheet.

The rear projection screen in the third aspect of the present inventionincludes the lenticular lens sheet in addition of the components of therear projection screen in the second aspect of the present invention.Thus, the rear projection screen in the third aspect of the presentinvention makes images visible from directions in a wide viewing angle.

Preferably, the lenticular lens sheet of the rear projection screen inthe third aspect of the present invention has a front surface, on theviewing side, coated with a light absorbing layer, and the contrastimproving sheet is bonded adhesively at least to the light absorbinglayer of the lenticular lens sheet. Thus, the rear projection screen hashigh rigidity even when the component lenses of the lenticular lenssheet are arranged at short pitches.

A rear projection screen in a fourth aspect of the present invention fordisplaying images by transmitting image light rays projected thereon andemitting the image light rays toward the viewing side includes: aFresnel lens sheet that deflects the image light rays obliquely incidenton the entrance surface thereof toward the viewing side in a directionsubstantially perpendicular to the exit surface thereof, the Fresnellens sheet including a total-reflection Fresnel lens formed on anentrance surface thereof, and the contrast improving sheet in the firstaspect of the present invention, the contrast improving sheet beingdisposed on the exit surface thereof; and a lenticular lens sheetdisposed on the viewing side of the Fresnel lens sheet to diffuse theimage light rays.

Since the rear projection screen in the fourth aspect of the presentinvention includes the Fresnel lens sheet provided with thetotal-reflection Fresnel lens on the entrance surface thereof, it ispossible to reduce stray light rays satisfactorily and prevent formationof double images due to mirror reflection effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary sectional view of a contrast improving sheet ina first embodiment of the present invention;

FIG. 2 is a ray tracing diagram showing paths followed by light rays inthe contrast improving sheet shown in FIG. 1;

FIG. 3 is a view of assistance in explaining an arrangement ofprotrusions formed in a first optical functional element included in thecontrast improving sheet in the first embodiment;

FIG. 4 is a view of assistance in explaining another arrangement ofprotrusions formed in a first optical functional element included in thecontrast improving sheet in the first embodiment;

FIG. 5 is a view of a laminated structure formed by combining a firstoptical functional element and a second optical functional elementincluded in the contrast improving sheet in the first embodiment;

FIG. 6 is a view of another laminated structure formed by combining afirst optical functional element and a second optical functional elementincluded in the contrast improving sheet in the first embodiment;

FIG. 7 is a view of a further laminated structure formed by combining afirst optical functional element and a second optical functional elementincluded in the contrast improving sheet in the first embodiment;

FIG. 8 is a perspective view of a rear projection screen provided withthe contrast improving sheet in the first embodiment;

FIG. 9 is a perspective view of another rear projection screen providedwith the contrast improving sheet in the first embodiment;

FIG. 10 is a fragmentary sectional view of a contrast improving sheet ina second embodiment of the present invention;

FIG. 11 is a perspective view of a rear projection screen provided withthe contrast improving sheet in the second embodiment;

FIG. 12 is a perspective view of another rear projection screen providedwith the contrast improving sheet in the second embodiment;

FIG. 13 is a ray tracing diagram showing light paths followed by lightrays in a contrast improving sheet in Example 1;

FIG. 14 is a diagram showing the light distribution characteristic ofthe contrast improving sheet in Example 1;

FIG. 15 is a diagrammatic view of assistance in explaining the opticalcharacteristic of a contrast improving sheet according to the presentinvention; and

FIG. 16 is a diagrammatic view of assistance in explaining the opticalcharacteristic of a conventional lenticular lens sheet.

EMBODIMENTS CARRYING OUT THE INVENTION

Preferred embodiment of the present invention will be described withreference to the accompanying drawings.

Contrast Improving Sheet

Referring to FIG. 1, a contrast improving sheet 11 in a first embodimentof the present invention is intended to be incorporated into a display,such as a rear projection display, a liquid crystal display or a plasmadisplay. The contrast improving sheet 11 includes a first opticalfunctional element 1 and a second optical functional element 2 attachedto the exit surface of the first optical functional element 1.

Referring to FIGS. 1 and 2, the first optical functional element 1 isprovided with a plurality of protrusions 3 having a trapezoidal crosssection and arranged in a direction or distributed in a plane. Each ofthe protrusions 3 has a pair of opposite total-reflection facets 5 fortotally reflecting image light rays 4 substantially perpendicularlyincident on the entrance surface of the first optical functional element1, and a flat facet 6 connecting the front edges, on the exit surface ofthe first optical functional element 1 contiguous with the secondoptical functional element 2, of the pair of total-reflection facets 5.V-grooves between the adjacent protrusions 3 are filled up with V-shapedlight absorbing layers 7 of a light absorbing material. Thetotal-reflection facets 5 of each protrusion 3 are inclined surfacesrepresented by inclined lines of a trapezoidal cross section and areboundaries between the protrusion 3 and the adjacent light absorbinglayers 7.

The protrusions 3 of the first optical functional element 1 may belinear protrusions extended with their ridges 12 extended in a directionand arranged in a vertical direction as shown in FIG. 3 or may besquare-prismoidal protrusions arranged in columns and rows as shown inFIG. 4.

As shown in FIGS. 1 and 2, the second optical functional element 2 isplaced in contact with the exit surface of the first optical functionalelement 1 including the flat facets 6 of the protrusions 3 and thesurfaces of the light absorbing layers 7. The second optical functionalelement 2 is provided with a light path correcting layer 9. The lightpath correcting layer 9 corrects light paths followed by image lightrays 4 totally reflected by the total-reflection facets 5 of theprotrusions 3 of the first optical functional element 1 such that theimage light rays 4 are emitted toward the viewing side substantiallyperpendicularly to the exit surface of the contrast improving sheet 11.The light path correcting layer 9 is formed by combining protrusions 15and 16 formed of two types of transparent materials respectively havingdifferent refractive indices, respectively. The protrusions 15 and 16having a triangular cross section demarcated by a boundary plane formedby arranging inclined planes 8 in a triangular shape in a sectionalview. Inclined planes 8 inclined in opposite directions, respectively,are arranged alternately. The light path correcting layer 9 refracts theimage light rays totally reflected by the total-reflection facets of theprotrusions 3 of the first optical functional element 1 such that theimage light rays 4 are emitted in a direction substantiallyperpendicular to the exit surface of the contrast improving sheet 11.

The inclined planes 8 in the light path correcting layer 9 of the secondoptical functional element 2 is formed so as to conform to the shape ofthe protrusions of the first optical functional element 1. Moreconcretely, the ribs 15 and 16 are extended horizontally and theinclined boundary planes 8 demarcating the ribs 15 and 16 extendhorizontally and parallel to the ridges 12 of the protrusions 3 as shownin FIG. 5 when the protrusions 3 are linear protrusion extended withtheir ridges 12 extended horizontally. The ribs 15 and 16 are extendedvertically and the inclined boundary planes 8 demarcating the ribs 15and 16 extend vertically and parallel to the ridges 12 of theprotrusions 3 as shown in FIG. 6 when the protrusions 3 are linearprotrusion extended with their ridges 12 extended vertically. When thesquare-prismoidal protrusions 3 are arranged in columns and rows, theprotrusions 15 and 16 are formed in a square-prismoidal shape and theinclined planes 8 between the protrusions 15 and 16 are arrangedsubstantially parallel to the total-reflection facets 5 of theprotrusions 3 as shown in FIG. 7.

The pitches P₁ of the protrusions 3 of the first optical functionalelement 1 and pitches P₂ of the inclined planes 8 of the light pathcorrecting layer 9 of the second optical functional element 2 meet acondition expressed by an inequality: P₁/P₂>3.5. Preferably, the pitchesP₁ and P₂ are determined selectively so as to meet a condition expressedby an inequality: P₁/P₂>3.5 and requiring that P₁/P₂ is not an integer.Those pitches are adjusted so as to meet the foregoing condition toprevent the formation of conspicuous moiré resulting from interferencebetween those periodic structures.

Light paths in the contrast improving sheet 11 in the first embodimentwill be described with reference to FIG. 2.

Indicated at A in FIG. 2 is an image light ray perpendicularly incidenton the entrance surface. The contrast improving sheet 11 in the firstembodiment is formed so as to meet a condition expressed by Expression(1):n ₃ /n ₂=cos 3θ₂/cos θ₂,  (1)where θ₂ is the inclination angle of the inclined planes 8 of the lightpath correcting layer 9 of the second optical functional element 2(angle between each of the inclined plane 8 and a normal to the sheetsurface of the second optical functional element 2), n₂ is therefractive index of a material forming a part of the light pathcorrecting layer 9 on the side of the first optical functional element 1with respect to the inclined planes 8 (protrusions 16), and n₃ is therefractive index of a material forming a part of the light pathcorrecting layer 9 on the opposite side of the front optical functionalelement 1 with respect to the inclined planes 8 (protrusions 15). Asobvious from Expression (1), θ₂<30° and n₃<n₂ in the second opticalfunctional element 2.

Thus, the image light ray A perpendicularly incident on the entrancesurface passes the flat facet 6 without being reflected by thetotal-reflection facet 5 of the protrusion 3; then it is totallyreflected by the inclined plane 8 of the light path correcting layer 9;then it is refracted at the inclined plane 8 opposed to the formerinclined plane; and then it is emitted from the second opticalfunctional element 2 perpendicularly to the exit surface of the secondoptical functional element 2, i.e., in a direction parallel to theincident image light ray.

Indicated at B is an image light ray perpendicularly incident on theentrance surface similarly to the image light ray A. The contrastimproving sheet in the first embodiment meets a condition expressed byan expression (2):n ₂·sin 2θ₂ =n ₁·sin 2θ₁,  (2)where θ₁ is the inclination angle of the total-reflection facets 5 ofthe protrusions 3 of the first optical functional element 1 (anglebetween each of the total-reflection facets 5 and a normal to the sheetsurface of the first optical functional element 1), n₁ is the refractiveindex of a material forming the protrusions 3 of the first opticalfunctional element 1, θ₂ is the inclination angle of the inclined planes8 of the light path correcting layer 9 of the second optical functionalelement 2 (angle between each of the inclined plane 8 and a normal tothe sheet surface of the second optical functional element 2), and n₂ isthe refractive index of a material forming a part of the light pathcorrecting layer 9 on the side of the first optical functional element 1with respect to the inclined planes 8 (protrusions 16). Thus, the imagelight ray B perpendicularly incident on the entrance surface is totallyreflected by the total-reflection facet 5 of the protrusion 3 of thefirst optical functional element 1; then it passes through the flatfacets 6; then it is refracted at the inclined plane 8 of the light pathcorrecting layer 9 of the second optical functional element 2; and thenit is emitted perpendicularly to the exit surface (in a directionparallel to the incident light ray). The image light rays A and Boutgoing from the second optical functional element 2 are parallel toeach other.

Expression (2) may be made insignificant by determining θ₁, θ₂, n₁ andn₂ such that θ₁=θ₂ and n₁=n₂ and the ratio n₃/n₂ may be selectivelydetermined so that only the condition expressed by Expression (1) may besatisfied. The image light rays A and B may be made unparallel to eachother to make the contrast improving sheet 11 exercise a weak diffusioneffect by determining the refractive indices n₁, n₂ and n₃ and theinclination θ₂ of the inclined planes 8 of the light path correctinglayer 9 such that the condition expressed by Expression (1) and/or thecondition expressed by Expression (2) is changed slightly.

The refractive index n₄ of the material (transparent resin) forming thelight absorbing layers 7 is smaller than the refractive index n₁ of thematerial forming the protrusions 3, and the inclination angle θ₁ of thetotal-reflection facets 5 of the protrusions, the refractive index n₁ ofthe material forming the protrusions 3, and the refractive index n₄ ofthe material forming the light absorbing layers 7 meet Expression (3):θ₁=cos⁻¹(n ₄ /n ₁)−sin⁻¹(1/2n₁).  (3)

The image light rays A and B perpendicularly incident on the entrancesurface are totally reflected by the total-reflection facets 5corresponding to the boundary planes between the protrusions 3 of thefirst optical functional element 1 and the light absorbing layers 7contiguous with the protrusions 3.

A light ray C obliquely incident on the entrance surface at an incidenceangle of about 30° are not totally reflected by the total-reflectionfacets 5 corresponding to the boundary planes between the protrusions 3of the first optical functional element 1 and the light absorbing layers7 contiguous with the protrusions 3, penetrates into and is absorbed bythe light absorbing layer 7. The light ray C is a stray light ray thatotherwise leaves the exit surface from a position different from thatfrom which the image light ray that follows a regular light path leavesthe exit surface. The light ray C generally causes a double image.

Preferably, the material (transparent resin) having a small refractiveindex and forming the light absorbing layers 7 contain dispersed,light-absorbing particles or colored with a light-absorptive dye toimprove contrast.

A light ray D is obliquely incident on the entrance surface at anincidence angle θ₄ smaller than that at which the light ray C isincident on the entrance surface. The light ray D, similarly to theimage light ray B; then it is totally reflected by the total-reflectionfacet 5 of the protrusion 3 of the first optical functional element 1;then it passes through the flat facets 6; then it is refracted at theinclined plane 8 of the light path correcting layer 9; and then it isemitted through the second optical functional element 2 at an emergenceangle θ₅ twice the incidence angle θ₄. Therefore, when diffused lightrays fall on the contrast improving sheet 11 at incidence angles θ₄ inthe range of about 5° to about 10°, the light rays are emitted from thecontrast improving sheet 11 at emergence angles θ₅ about twice theincidence angle θ₄ and directivity can be slightly reduced and theemergence angle can be slightly increased.

A light ray E is an external light ray perpendicularly incident on theexit surface. The light ray E is refracted at the inclined boundaryplane 8 of the light path correcting layer 9 of the second opticalfunctional element 2. It is not totally reflected at the boundarybetween the first optical functional element 1 and the second opticalfunctional element 2, and penetrates into and is absorbed by the lightabsorbing layer 7. Light paths followed by external light rays, notshown, incident on the exit surface at incidence angles, for example,not smaller than 50° are bent in the direction of thickness of the sheetby the inclined boundary planes 8 of the light path correcting layer 9of the second optical functional element 2. Consequently, those externallight rays are not totally reflected, and penetrate into and is absorbedby the light absorbing layers 7.

Thus, the contrast improving sheet 11 in the first embodiment is formedby combining the first optical functional element 1 and the secondoptical functional element 2 that meet the conditions expressed byExpressions (1) to (3). Therefore, the light rays A and B (image lightrays) substantially perpendicularly incident on the entrance surface ofthe first optical functional element 1 can be emitted substantiallyperpendicularly to the exit surface regardless of the position ofincidence on the entrance surface, and the light ray C (stray light ray)obliquely incident on the entrance surface of the first opticalfunctional element 1 and the light ray E (external light ray) incidenton the exit surface of the second optical functional element 2 can beabsorbed by the light absorbing layers 7 of the first optical functionalelement 1. Consequently, contrast is improved and the image light raysincident on the entrance surface can be transmitted toward the viewingside without loss in high directivity.

The contrast improving sheet 11 in the first embodiment can befabricated by individually forming the first optical functional element1 to be disposed on the back side and the second optical functionalelement 2 to be disposed on the viewing side, and bonding together thefirst optical functional element 1 and the second optical functionalelement 2. Since the pitches P₁ of the protrusions 3 of the firstoptical functional element 1 and the pitches P₂ of the inclined boundaryplanes 8 of the light path correcting layer 9 of the second opticalfunctional element 2 are selectively determined so as to meet acondition expressed by an inequality: P₁/P₂>3.5, preferably, the pitchesP₁ and P₂ are determined selectively so as to meet a condition expressedby an inequality: P₁/P₂>3.5 and requiring that P₁/P₂ is not an integer,work for the positional adjustment of the first optical functionalelement 1 and the second optical functional element 2 relative to eachother is not necessary and hence the contrast improving sheet 11 in thefirst embodiment can be easily fabricated.

In a process of forming the first optical functional element 1, theplurality of protrusions 3 are formed by a known method using a mold,such as a hot pressing method, a thermal polymerization method or aradiation curing method, and then the V-grooves between the protrusions3 are filled up with the light absorbing material by a wiping method orthe like to form the light absorbing layers 7.

In a process of forming the second optical functional element 2, theprotrusions on the side of the exit surface, namely, the protrusions 15(or the protrusions on the side of the entrance surface, namely, theprotrusions 16) are formed by the same method as that of forming theprotrusions 3 of the first optical functional element 1, and then theV-grooves between the adjacent protrusions on the side of the exitsurface, namely, the protrusions 15 (or the protrusions on the side ofthe entrance surface, namely, the protrusions 16) are filled up with aradiation-curable resin or the like. The radiation-curable resin fillingup the V-grooves is cured to complete the other protrusions, namely, theprotrusions 16 (or the protrusions 15). In the method, the secondoptical functional element 2 may be fabricated by forming theprotrusions on the side of the exit surface, namely, the protrusions 15,by molding, then filling up the V-grooves between the adjacentprotrusions on the side of the exit surface, namely the protrusions 15,with the resin for forming the protrusions on the side of the entrancesurface, namely, the protrusions 16, then attaching the first opticalfunctional element 1 to the resin filling up the V-grooves, and curingthe resin filling up the V-groove to complete the protrusions on theside of the entrance surface, namely, the protrusions 16, of the secondoptical functional element 2. Thus, the first optical functional element1 and the second optical functional element 2 can be bonded together inthe process of forming the second optical functional element 2. Anothermethod may form the protrusions 3 of the first optical functionalelement 1 and the light path correcting layer 9 of the second opticalfunctional element 2 on substantially transparent films or sheets, andmay bond together the films or the sheets with an adhesive layer.

It is preferable from the viewpoint of manufacture that at least thematerial for forming the first optical functional element 1 and at leasteither of two types of materials for forming the protrusions 15 and 16of the light path correcting layer of the second optical functionalelement 2 are radiation-curable resins. Possible radiation-curableresins are those generally used in this field including, for example,UV-curable resins and electron beam-curable resins, such as acrylicresins, epoxy resins and urethane resins. Possible films as thetransparent films or sheets on which the protrusions 3 of the firstoptical functional element 1 and the light path correcting layer 9 ofthe second optical functional element 2 are formed are generally usedfilms including polyester films and polycarbonate films.

The light absorbing layers 7 of the first optical functional element 1are formed of a material prepared by dispersing light absorbingparticles in a transparent resin having a small refractive index orcoloring such a resin with a light absorbing dye. Although it ispreferable that the light absorbing layers 7 have an achromatic color,such as black or grey, the light absorbing layers 7 may be formed of amaterial capable of absorbing specific wavelengths dependent on thecharacteristics of image light. Suitable light absorbing particles arethose of carbon black, graphite, metal salts, such as black iron oxide,colored organic materials and colored glass. Suitable dyes are organicxanthene dyes, such as acid red, and organic neodymium dies, such asneodymium carboxylate.

The material for forming the protrusions 3 of the first opticalfunctional element 1 and the two types of materials forming theprotrusions 15 and 16 of the light path correcting layer 9 of the secondoptical functional element 2, and at least either of the transparentfilms or sheets on which the protrusions 3 of the first opticalfunctional element 1 and the light path correcting layer 9 of the secondoptical functional element 2 are formed may contain a coloring matterfor the further improvement of contrast.

The contrast improving sheet 11 in the first embodiment may be furtherprovided with a UV absorbing layer (or a film or a sheet) laminated tothe exit surface of the second optical functional element 2. At leastone of a hard coating layer, an antireflection layer, a glare-prooflayer and an antistatic layer may be attached to the exit surface of thecontrast improving sheet 11 in the first embodiment.

The contrast improving sheet 11 in the first embodiment can be disposedin front of an image forming surface, facing the viewing side, of adisplay, such as a rear projection display, an electron-flood-beam flatdisplay (a liquid crystal display, a plasma display or an EL display).When the contrast improving sheet 11 is used in combination with a rearprojection display, the contrast improving sheet 11 is particularlyeffective in improving contrast in images under external light and inimproving images by effectively absorbing optical noise, such as lightforming ghosts and rainbows due to the action of a Fresnel lens and backmirror combined with the rear projection display.

Rear Projection Screen

A rear projection screen provided with the contrast improving sheet 11in the first embodiment will be described. The rear projection screen issuitable for use in a rear projection display.

FIG. 8 shows a rear projection screen 21A provided with the contrastimproving sheet 11 in the first embodiment by way of example. Referringto FIG. 8, the rear projection screen 21A transmits image light raysprojected on its entrance surface and emits the image light rays throughits exit surface toward the viewing side to display an image. The rearprojection screen 21A includes a Fresnel lens sheet 22 for deflectingthe image light rays projected on its back surface in a directionsubstantially perpendicular to the exit surface, and the contrastimproving sheet 11 disposed on the front side of the Fresnel lens sheet22. The contrast improving sheet 11 includes, in addition to the firstoptical functional element 1 and the second optical functional element2, a layer 19, such as a UV absorbing layer.

The rear projection screen 21A shown in FIG. 8 is not provided with anyoptical element for diffusing image light, such as a lenticular lenssheet. If the image light rays incident on the contrast improving sheet11 is diffused light rays, the contrast improving sheet 11 is able tosubstantially double the angle of diffusion (angle θ₄ in FIG. 2).Therefore, the rear projection screen 21A shown in FIG. 8 is able toexercise a diffusing effect of about 10° with respect to verticaldirections and about 20° with respect to horizontal directions if theFresnel lens sheet 22 contains a light diffusing agent such that lightrays traveled through the Fresnel lens sheet 22 are diffused in adiffusing angle of about 10°.

FIG. 9 shows another rear projection screen 21B provided with thecontrast improving sheet 11 in the first embodiment. Referring to FIG.9, the rear projection screen 21B is a three-layer screen including aFresnel lens sheet 22 for deflecting the image light rays projected onits back surface in a direction substantially perpendicular to the exitsurface, a lenticular lens sheet 23 disposed on the front side of theFresnel lens sheet 22 to diffuse image light rays, and the contrastimproving sheet 11 disposed on the front side of the lenticular lenssheet 23. The contrast improving sheet 11 includes, in addition to thefirst optical functional element 1 and the second optical functionalelement 2, a layer 19, such as a UV absorbing layer.

Since the contrast improving sheet 11 in the first embodiment is able todiffuse image light rays incident on its entrance surface in aboutdouble diffusing angle, the amount of the light absorbing agentcontained in the lenticular lens sheet 23 can be reduced and,consequently, the rear projection screen 21B, as compared with theconventional rear projection screen, is capable of displaying clearerimages.

The contrast improving sheet 11 and the lenticular lens sheet 23 may bebonded together with an adhesive layer. When the lenticular lens sheet23 is a fine, thin one, it is preferable to bond the contrast improvingsheet 11 to a front light absorbing layers (black stripes) 23 b of thelenticular lens sheet 23 to increase the rigidity of the sheet. When theimage source of a rear projection display is a single-tube light source,such as a LCD or a DLP, and the exit surface of the lenticular lenssheet 23 included in the rear projection screen 21B is flat, thecontrast improving sheet 11 may be bonded to the entire front surface ofthe lenticular lens sheet 23.

As shown in FIG. 9, the lenticular lens sheet 23 is provided on its backsurface with a plurality of vertical lenticular lenses 23 a, and theprotrusions 3 of the contrast improving sheet 11 are extendedperpendicularly to the vertical lenticular lenses 23 a.

Although the contrast improving sheet 11 and the Fresnel lens sheet 22are individual members in each of the rear projection screen 21A shownin FIG. 8 and the rear projection screen 21B shown in FIG. 9, it ispossible to use a total-reflection Fresnel lens sheet 31 as shown inFIG. 10, which is integrally provided with a functional structure havingthe function of a contrast improving sheet.

As shown in FIG. 10, the total-reflection Fresnel lens sheet 31 fordeflecting image light rays obliquely incident on its entrance surfacein a direction substantially perpendicular to its exit surface includes:total-reflection Fresnel lens 32 formed in its entrance surface, and thecontrast improving sheet including the first optical functional element1 and the second optical functional element 2, both elements beingformed integrally with the Fresnel lens 32.

Light rays F, namely, image light rays, incident on the entrance facets33 of the total-reflection Fresnel lens 32 and totally reflected by thetotal-reflection facets 34 of the total-reflection Fresnel lens 32 fallon the first optical functional element 1 substantially perpendicularly.Consequently, the light rays passed through the first optical functionalelement 1 and the second optical functional element 2 are emitted fromthe total-reflection Fresnel lens sheet 31 substantially perpendicularlyto the exit surface of the Fresnel lens sheet 31. Light rays G incidenton the entrance facets 33 of the total-reflection Fresnel lens 32 formedin the entrance surface, not totally reflected by the total-reflectionfacets 34 and traveling in stray light rays fall obliquely on the firstoptical functional element 1 and are absorbed by the light absorbinglayers 7. Thus, stray light rays are not emitted toward the viewingside.

The total-reflection Fresnel lens sheet 31 is included in a rearprojection screen 21C as shown in FIG. 11. Referring to FIG. 11, therear projection screen 21C includes the total-reflection Fresnel lenssheet 31 and a lenticular lens sheet 24 disposed on the front side ofthe total-reflection Fresnel lens sheet 31. The total-reflection Fresnellens sheet 31 can be included in a rear projection screen 21D as shownin FIG. 12. As shown in FIG. 12, the rear projection screen 21D includesthe total-reflection Fresnel lens sheet 31 and a total-reflectionlenticular lens sheet 25 disposed in front of the total-reflectionFresnel lens sheet 31.

EXAMPLES

Examples of the foregoing preferred embodiments will be described.

Example 1

Protrusions of 0.6 mm in height having a trapezoidal sectional shapewere formed of a UV-curable resin having a refractive index n₁ f 1.55after curing on one of the major surfaces of a 50 μm thick PET filmhaving a refractive index of 1.6. The protrusions were arranged atpitches P₁ of 0.3 mm. Each protrusions had a flat facet of 0.169 mm inwidth and total-reflection facets inclined at an inclination angle θ₁,of 8°. light absorbing layers were formed by filling up V-groovesbetween the adjacent protrusions having the trapezoidal sectional shapewith a light absorbing material prepared by dispersing black beadshaving a mean particle size of 6 μm in a acrylic coating material havinga refractive index n₄ of 1.49 to form a first optical functionalelement.

Protrusions of 135 μm in height having a triangular sectional shape wereformed of a resin produced by modifying an acrylic resin and having arefractive index n₃ of 1.48 on one of the major surfaces of a 1.5 mmthick acrylonitrile-styrene copolymer film having a refractive index of1.53. The protrusions were arranged at pitches P₂ of 40 μm (P₁/P₂=7.5)so that conspicuous moiré may not be formed by the protrusions. Theinclination angle θ₂ of the facets of the protrusions was 8° equal tothat of the inclination angle of the facets of the protrusions 3 of thefirst optical functional element 1. An EB-curable styrene resin wasfilled in V-grooves between the protrusions and the EB-curable styreneresin was cured to form a second optical functional element 2.

The first and second optical functional elements thus fabricated werebonded together with their protrusions opposed to each other to completea contrast improving sheet in Example 1.

(Evaluation)

The contrast improving sheet in Example 1 was evaluated.

A commercial rear projection television set (Hitachi) on the market wasused for evaluating the contrast improving sheet. The rear projectiontelevision set was provided with a three-layer rear projection screenconsisting of a Fresnel lens sheet, a lenticular lens sheet and acolored sheet, namely, a lightly colored transparent resin sheet. A testrear projection screen was built by using the contrast improving sheetin Example 1 instead of the colored sheet of the rear projection screen.Television images were displayed on the test rear projection screen andthe conventional rear projection screen and were observed forcomparison. The first and the second optical functional element of thecontrast improving sheet were combined and the contrast improving sheetwas incorporated into the test rear projection screen as shown in FIG.9.

The rear projection screen provided with the contrast improving sheet inExample 1, as compared with the conventional rear projection screenprovided with the colored sheet, displayed images in improved contrastand could display clear images even in a light environment. Where as afaint rainbow was observed on the conventional rear projection screenprovided with the colored sheet due to the agency of the Fresnel lenssheet, any rainbow could not be observed at all on the rear projectionscreen provided with the contrast improving sheet in Example 1.

FIG. 13 is a ray tracing diagram showing light paths followed by lightrays in a contrast improving sheet in Example 1. FIG. 14 is a diagramshowing the light distribution characteristic of the contrast improvingsheet in Example 1. As obvious from FIGS. 13 and 14, the contrastimproving sheet in Example 1 has excellent directivity. Gain G mentionedin FIG. 14 was calculated by using an expression:G=(Luminance)/(Illuminance) expressing the relation between luminanceand illuminance.

Example 2

A contrast improving sheet in Example 2 was fabricated by the samemethod as that of forming the contrast improving sheet in Example 1 andusing the same materials as those used for forming the contrastimproving sheet in Example 1, except that protrusions of a first opticalfunctional element and a back part, contiguous with the first opticalfunctional element, of a second optical functional element were formedof the same resin having a refractive index of 1.55, and a front part,apart from the first optical functional element, of the second opticalfunctional element was formed of a silicone resin having a refractiveindex of 1.43.

(Evaluation)

The contrast improving sheet in Example 2 was evaluated.

A rear projection television set having a projection distance of 300 mm,a projection angle of 65° and a screen size of 60 in. was used as a testrear projection television set. The test rear projection television setwas provided with a conventional rear projection screen including atotal-reflection Fresnel lens sheet and a lenticular lens sheet. A testrear projection screen was provided with a composite total-reflectionFresnel lens sheet built by bonding the contrast improving sheet inExample 2 to the front surface of a total-reflection Fresnel lens sheetof the same optical characteristic as the total-reflection Fresnel lenssheet of the conventional rear projection screen instead of thetotal-reflection Fresnel lens sheet of the conventional rear-projectionscreen. Television images displayed on the conventional rear projectionscreen and the test rear projection screen provided with the contrastimproving sheet in Example 2 were observed for comparison. First andsecond optical functional elements of the contrast improving sheet werelaminated and the contrast improving sheet was incorporated into therear projection screen as shown in FIG. 11.

The test rear projection screen provided with the composite Fresnel lenssheet formed by bonding together the Fresnel lens sheet and the contrastimproving sheet in Example 2, as compared with the conventional rearprojection screen, displayed images in improved contrast and moreclearly in a light environment. Whereas double images due to the agencyof the total-reflection Fresnel lens sheet and those due to mirrorreflection were observed on the conventional rear projection screen, anydouble images were not observed at all on the test rear projectionscreen.

Example 3

A contrast improving sheet in Example 3 was fabricated similarly to thecontrast improving sheet in Example 2, except that the contrastimproving sheet in Example 3 included a first optical functional elementhaving protrusions of 0.126 mm arranged at pitches P₁ of 0.063 mm and asecond optical functional element having protrusions of 60 μm arrangedat pitches P₂ of 18 μm (P₁/P₂=3.5).

(Evaluation)

The contrast improving sheet in Example 3 was evaluated.

The contrast improving sheet in Example 3 was combined with a liquidcrystal monitor of 15 in. in screen size included in a personal computer(PC). Images displayed on the screen of the liquid crystal monitor wereobserved through the contrast improving sheet in Example 3. First andsecond optical functional elements of the contrast improving sheet werelaminated and the contrast improving sheet was incorporated into theliquid crystal monitor as shown in FIG. 6.

The liquid crystal monitor reflected nothing in a light environment.Whereas clear images could be recognized in high contrast when thescreen of the liquid crystal monitor was viewed from a directionperpendicular to the liquid crystal monitor, any images could not beseen on the screen of the liquid crystal monitor when the screen of theliquid crystal monitor was viewed obliquely from horizontal directionsat 25° or greater to a normal to the screen of the liquid crystalmonitor. Thus, the satisfactory stealthy peep preventing function of thecontrast improving sheet was assured.

1. A contrast improving sheet to be used in combination with a display,said contrast improving sheet comprising an optical element adapted toemit image light rays substantially perpendicularly incident on anentrance surface thereof in a direction substantially perpendicular toan exit surface thereof regardless of incidence position on the entrancesurface and to absorb stray light rays obliquely incident on theentrance surface and external light fallen on the exit surface, whereinthe optical element comprises: a first optical functional elementprovided with a plurality of protrusions arranged in a direction ordistributed in a plane, each of the protrusions having a pair ofopposite total-reflection facets for totally reflecting image light rayssubstantially perpendicularly incident on an entrance surface thereofand a flat facet extending between front edges, on a side of an exitsurface thereof, of the pair of opposite total-reflection facets, andlight absorbing layers formed between the adjacent protrusions; and asecond optical functional element attached to a surface, on a viewingside, of the first optical functional element, and provided with a lightpath correcting layer for correcting light paths followed by the imagelight rays totally reflected by the total-reflection facets of theprotrusions such that the image light rays are emitted through an exitsurface thereof substantially perpendicularly to the exit surface. 2.The contrast improving sheet according to claim 1, wherein the lightpath correcting layer of the second optical functional element has aplurality of inclined surfaces formed in an alternate arrangement andadapted to emit the image light rays totally reflected by thetotal-reflection facets of the first optical functional element in adirection substantially perpendicular to the exit surface.
 3. Thecontrast improving sheet according to claim 2, characterized in that theinclined surfaces included in the light path correcting layer of thesecond optical functional element are formed such that substantiallyparallel are both image light rays substantially perpendicularlyincident on an entrance surface thereof, traveled through the flatfacets without being reflected by the total-reflection facets of theprotrusions of the first optical functional element, reflected by theinclined surfaces and refracted at the inclined surfaces, and imagelight rays totally reflected by the total-reflection facets of theprotrusions of the first optical functional element, traveled throughthe flat facets and refracted at the inclined surfaces.
 4. The contrastimproving sheet according to claim 1, characterized in that the lightabsorbing layers of the first optical functional element are formed of atransparent material having a refractive index smaller than that of amaterial forming the protrusions of the first optical functionalelement, the light absorbing layers containing light absorbingparticles.
 5. The contrast improving sheet according to claim 1,characterized in that the light absorbing layers of the first opticalfunctional element are formed of a colored material having a refractiveindex smaller than that of the material forming the protrusions of thefirst optical functional element.
 6. The contrast improving sheetaccording to claim 1, characterized in meeting a condition expressed byan expression: n₃/n_(2=cos) 3θ₂/cos θ₂, where θ₂ is an inclination angleof the inclined surfaces of the light path correcting layer of thesecond optical functional element (angle between each of the inclinedsurfaces and a normal to a sheet surface of the second opticalfunctional element), n₂ is a refractive index of a material forming apart of the light path correcting layer on a side of the first opticalfunctional element with respect to the inclined surfaces, and n₃ is arefractive index of a material forming a part of the light pathcorrecting layer on the opposite side of the first optical functionalelement with respect to the inclined surfaces.
 7. The contrast improvingsheet according to claim 1, characterized in meeting a conditionexpressed by an expression: n₂·sin 2θ_(2=n)1·sin 2θ₁, where θ₁ is aninclination angle of the total-reflection facets of the protrusions ofthe first optical functional element (angle between each of thetotal-reflection facets and a normal to a sheet surface of the firstoptical functional element), n₁ is the refractive index of a materialforming the protrusions of the first optical functional element, θ₂ isan inclination angle of the inclined surfaces of the light pathcorrecting layer of the second optical functional element (angle betweeneach of the inclined surfaces and a normal to a sheet surface of thesecond optical functional element), and n₂ is a refractive index of amaterial forming a part of the light path correcting layer on the sideof the first optical functional element with respect to the inclinedsurfaces.
 8. A rear projection screen for displaying images bytransmitting image light rays projected thereon and emitting the imagelight rays toward a viewing side, said rear projection screencomprising: a Fresnel lens sheet that deflects image light rays incidenton an entrance surface thereof toward a viewing side in a directionsubstantially perpendicular to an exit surface thereof; and a contrastimproving sheet as set forth in claim 1,the contrast improving sheetbeing disposed on the viewing side of the Fresnel lens sheet.
 9. A rearprojection screen for displaying images by transmitting image light raysprojected thereon and emitting the image light rays toward a viewingside, said rear projection screen comprising: a Fresnel lens sheet thatdeflects image light rays incident on an entrance surface thereof towarda viewing side in a direction substantially perpendicular to an exitsurface thereof; a lenticular lens sheet disposed on the viewing side ofthe Fresnel lens sheet to diffuse the image light rays; and a contrastimproving sheet as set forth in claim 1, the contrast improving sheetbeing disposed on a viewing side of the lenticular lens sheet.
 10. Therear projection screen according to claim 9, characterized in that thelenticular lens sheet has a front surface, on a viewing side, coatedwith a light absorbing layer, and the contrast improving sheet is bondedadhesively at least to the light absorbing layer of the lenticular lenssheet.
 11. A rear projection screen for displaying images bytransmitting image light rays projected thereon and emitting the imagelight rays toward a viewing side, said rear-projection screencomprising: a Fresnel lens sheet that deflects image light raysobliquely incident on an entrance surface thereof toward a viewing sidein a direction substantially perpendicular to an exit surface thereof,the Fresnel lens sheet including a total-reflection Fresnel lens formedon an entrance surface thereof and a contrast improving sheet as setforth in claim 1, the contrast improving sheet being disposed on an exitsurface thereof; and a lenticular lens sheet disposed on the viewingside of the Fresnel lens sheet to diffuse the image light rays.